Re-enable alias redefinition

alias modification introduced in a4c646f prevents to redefine already defined functions by using alias.

I find it disturbing because most of the time for simple functions (like "ll" for "ls -l") it is way faster to just redefine it instead of finding which of "functions" or "funced" must be used (I never know which one applies for which case), after that find the good option,...

I think fish should just overwrite the existing function, just like it was before and like every other shell does.